1. Field of the Invention
The invention relates to a method of examining inorganic materials which have been treated with organosilicon compounds.
2. Discussion of the Background
It is known that hydrophobicization of inorganic materials, in particular mineral building materials, is generally carried out using organosilicon compounds in the form of aqueous emulsions, microemulsions or dissolved in organic solvents (Product Information from Huls AG: "Anwendungen von organofunktionellen Silanen--DYNASYLAN.RTM.", 15.01.002/07.94/gu/100; "Bautenschutz mit Alkylsilanen--DYNASYLAN.RTM. BSM", 15.01.006/09.90/gu; Ch. Fliedner, "Silane als Hydrophobierungsmittel fur Beton", special publication 15.07.023 of Huls AG, Parts I and II have appeared in: Bautenschutz+Bausanierung 3/94 and 4/94). On the surface of the inorganic materials, right into the interior of a porous material, reactive groups of the organosilicon compounds form chemical bonds with the hydroxyl groups of the material; in parallel thereto, crosslinking reactions of the silicon compounds occur in the presence of water, which finally leads to a hydrophobicizing film chemically anchored to the surface and even in the pores, which film suppresses or reduces the destructive absorption and transport of water. Examples of building materials which may be mentioned here are concrete, sandstone, lime-sand brick, bricks and mortar. Pulverulent and granular materials such as aluminum oxide, titanium dioxide, magnesium oxide, glass powder and quartz powder, hydroxides, silicates and other mineral products and also glass microspheres are also often surface-modified with organosilicon compounds to improve their application properties.
At present, the testing of surface protection of building materials is carried out by means of water absorption (Technical Testing Instructions for Surface-Protection Systems, 1990 edition, Bulletin, Document No.: B 5234 --Test 90.1, page 30), the impregnation depth (exposure of a point to a depth of .gtoreq.1 cm and spraying with water), the absorbency profile (water absorption on successively sawn-off slices, M. Roth: Das Wassersaugprofil einer Siliconimpragnierung (The Water Absorbency Profile of a Silicone Impregnation), Bautenschutz und Bausanierung, 11, (1988), pages 43-45), pressure water storage (DIN 52 103) or absorption water storage (DIN 52 617). Decisive factors for the quality of the hydrophobicization are the chemical structure of the hydrophobicizing agent, the amount of active substance absorbed on the surface and in deeper zones and the chemical anchoring of the hydrophobicizing agent on the internal and external surfaces of the building material; virtually no information can be obtained about these factors using the above-mentioned tests, since only the overall water-repellent effect is measured.
Various analytical methods, such as TOF-SIMS (time-off-flight secondary-ion mass spectrometry), ESCA (electron-spectroscopy for chemical analysis), DRIFT (diffuse reflectance infrared Fourier transform) and Auger-electron spectroscopy, have been developed for examining surfaces. Apart from the complex equipment, the large amount of time required and the huge costs, these methods have the disadvantage that usually only the uppermost surface of the test material, in the nanometer range can be measured.
The dispersive IR (infrared spectroscopy) technique is also unsuitable for characterizing silane hydrophobicizing agents under conditions close to practice (Ghosh, S. and Handoo, S. K.: Infrared and Raman Spectral Studies in Cement and Concrete, Cement and Concrete Research, 10, (1980), pages 771-782). However, the penetration depth of hydrophobicizing agents in concrete could be determined by FT(Fourier transform) IR spectroscopy (A. Gerdes, T. Muller and F. H. Wittmann in: Werkstoff-wissenschaften und Bausanierung, Part 1, Symposium Report of the 3rd International Colloquium, edited by F. H. Wittmann, expert publishers (Kontakt & Studium Vol. 420) Ehningen bei Bobblingen, 1993, pages 460-475). Concrete flour milled to analytical fineness is mixed with potassium bromide (1:25) and pressed at 250 bar to give transparent tablets and an FT-IR spectrum was recorded thereon in the absorption range of 2900-2940 cm.sup.-1. The CH.sub.2 stretching vibration in the region of 2930 cm.sup.-1 serves for the qualitative or semiquantitative determination of hydrophobicizing agents based on organosilicon compounds, provided that organic compounds which can interfere with the detection are not present. Apart from the complication in terms of equipment and preparation, this method has further disadvantages: the various hydrophobicizing or modifying agents cannot be differentiated; active substance contents below about 0.2% cannot be measured; large particles cannot be analyzed without fine milling; hydrophobicizing agent mixtures are not recognizable as such; covalent fixing and physical coating cannot be differentiated; modifications of, for example, glass beads, aluminum oxide or silicates cannot be detected owing to a lack of sensitivity of the method. .sup.29 Si-MAS-NMR spectroscopy (29-Si magic angle spinning nuclear magnetic resonance spectroscopy) is, due to the high cost and the limited detection sensitivity, also unsuitable for quality testing when using the hydrophobicizing and modifying agents in day-to-day practice; rather, this method is used for fundamental studies (J. Grobe, K. Stoppek-Langner, W. Muller-Warmuth, S. Thomas, A. Benninghoven and B. Hagenhoff: Nachr. Chem. Tech. Lab., 41, (1993), No. 11, pages 1233-1240; B. Hagenhoff, A. Benninghoven, K. Stoppek-Langner and J. Grobe: Adv. Mater., (1994), 6, No. 2, pages 142-144; K. Albert, R. Brindle, J. Schmid, B. Buszewski and E. Bayer: Chromatographia, 38, (1994), pages 283-290).
The pyrolysis of silane-coated fillers, specifically talc, has been utilized for detecting organofunctional silanes used for coating, by pyrolyzing a relatively large amount of filler (10 g) at 720.degree. C. for about 30 minutes; the large amount was necessary to achieve the detection limits of 0.01-0.1% and the long pyrolysis time associated with the "large" amount of sample was necessary to favor secondary reactions (artifact formation) which were to aid the differentiation of individual silanes. The pyrolysis products were collected in an organic solvent (triethylene glycol dimethyl ether) at 0.degree. C. and the absorption solution together with the condensate, which can separate out before the absorber, was subsequently analyzed by gas chromatography (off-line); separation column: 60 m DB wax thin-film capillary, film thickness: 0.5 .mu. (R. Doiber and N. Wamser: Fresenius J. Anal. Chem., 342, (1992), pages 381-386). Disadvantages of this method are that reaction products which are characteristic or selective for particular coating agents (ammonia, hydrogen sulphide, acetone) are formed only in exceptional cases and that the major part of the numerous indeterminable pyrolysis products is identical for all silanes, so that only quantitative differences in the gaschromatographic peak distribution can be used for identification. In addition, the alkoxy groups (methoxy or ethoxy groups) characteristic of many hydrophobicizing and modifying agents cannot be detected.
The very complicated and additionally high-cost FT-IR analytical method (pyrolysis gas chromatography with Fourier transform infrared spectroscopy) has been used to establish whether N-phenyl-1-aminopropyltrimethoxysilane in acetic acid is chemically bound or is able to be washed off with methanol in the silane treatment of E glass fibers (N. Ikuta, T. Hori, H. Naitoh, Y. Kera, E. Nishio and I. Abe, Compos. Interfaces (1993), 1(6), 455-462).
Pyrolysis gas chromatography has also been used to examine poly(diorganosiloxanes) (S. Fujimoto, H. Ohtani and S. Tsuge: Fresenius Z. Anal. Chem., 331, (1988), pages 342-350), where numerous cyclic oligomers were mainly formed and these were separated on a thin-film capillary (WCOT column). The quantitative analysis of these oligomers, made possible a determination of the original composition by means of the components, of which there were often more than 19, only by means of a complicated method.
To assess the quality of the hydrophobicization or surface modification of inorganic materials there is a need for a method of examination by means of which the organosilicon compounds used here can be detected as specifically as possible and their concentration on the surface and possibly also at various depths of the inorganic substrate can be determined quantitatively.
It is therefore an object of the invention to provide a method which makes it possible, with justifiable expense, to examine inorganic materials which have been treated with organosilicon compounds for hydrophobicization or surface modification, to identify the organosilicon compounds used very reliably and also to be able to quantify them.
It has surprisingly been found that organosilicon hydrophobicizing and modifying agents on inorganic materials, in particular on and in building materials, can be specifically detected and reproducibly determined quantitatively by, generally, first pyrolyzing the sample material over a period of a few seconds and analyzing the pyrolysis products on-line by gas chromatography on thin-layer capillaries (PLOT (porous layer open tubular) columns, i.e. capillary column and porous layer). A main component preferentially formed in the pyrolysis is conveniently used for identification and quantitative determination.